There are many applications today where communication system receivers, transmitters, power amplifiers and other elements are interconnected by electrical cables carrying radio frequency (RF) signals. For convenience of description, the combination of a receiver and transmitter is referred to herein as a “transceiver”, abbreviated as “T/R”. For proper operation the signal losses occurring in these cables must be taken into account in designing and constructing the systems. If the size and/or configuration of the installations vary from application to application, then the cable losses will likely also vary and must therefore be adjusted or compensated for each system installation. In the aviation industry for example, various standards have been adopted to attempt to limit the variability encountered in such system installations. A non limiting example is in the installation in aircraft of satellite communication systems for use with the Inmarsat® satellites.
FIG. 1 is a simplified electrical schematic block diagram of airborne satellite communication system 20 according to the prior art, suitable for use with the Inmarsat satellites, which operate for example at frequencies in the range of 1,626.5 to 1,660.5 mega-Hertz, but such frequencies are not critical to the present invention. System 20 comprises transceiver (T/R) 22 coupled by RF pathway 23 to high power amplifier (HPA) 24. HPA 24 is coupled by RF pathway 25 to diplexer 26. Diplexer 26 is coupled by RF pathway 27 to antenna 30 and by RF pathway 29-1 to low noise amplifier (LNA) 28. LNA 28 is coupled by RF pathway 29-2 to T/R 22. LNA 28 may be combined with diplexer 26 so that only a single pathway (hereafter RF pathway or link 29) is needed. Either arrangement is useful. Diplexer 26 is conventional and separates the incoming and outgoing RF signals. Incoming RF signals received from antenna 30 are directed by diplexer 26 to LNA 28 where they are amplified and sent over RF link 29 to T/R 22 where they are demodulated and/or decoded and the results presented to the user in audio or other form via communication link 32. Similarly, outgoing communications received from the user via link 32 are modulated and/or encoded by T/R 22 to form a modulated and/or encoded RF signal that is sent via RF link 23 to HPA 24 where it is amplified and sent via RF link 25 to diplexer 26, which in turn directs it to antenna 30 over RF link 27. Elements 22, 24, 26, 28 and 30 of RF communication system 20 are conventional and well known in the art.
HPA 24 is typically physically located close to diplexer 26 and antenna 30 to minimize loss of signal power over link 25. However, T/R unit 22 may be near or far from HPA 24 depending upon the size and configuration of the aircraft Thus signal losses in, for example, link 23 can be a serious concern. To accommodate this installation variability, a standard has been adopted in the aviation industry requiring that transceiver (T/R) 22 deliver a power level sufficient to overcome up to 25 dB of cable loss in link 23 and still provide adequate drive at input 24-1 of HPA 24. A lower limit of 19 dB of cable loss is also specified to minimize the dynamic range that is required at input 24-1 to HPA 24. If the actual loss along RF cable or link 23 for a particular installation is less than the 19 dB minimum, then additional loss must be inserted in the cabling to force the signal arriving at HPA 24 to conform to the 19-25 dB loss range specified in the standard. One or more fixed or manually settable attenuators 34 are provided at input 24-1 of HPA 24 or in RF cable or link 23 between T/R 22 and HPA 24 to adjust the RF signal loss along link 23 to meet the desired specification, for example, 19-25 dB total loss in the case of Inmarsat communication systems. Attenuator(s) 34 are set to the necessary attenuation during system design and installation and generally depend upon the aircraft size and configuration. Attenuator(s) 34 will often vary from installation to installation and aircraft to aircraft because of differences in aircraft size and wiring configuration.
These additional attenuators and/or other custom components add weight, increase installation time and reduce overall system reliability due to the extra cable connectors and fittings that may loosen or degrade over time. They also make system maintenance more complex and expensive since different aircraft in the same fleet may have different attenuator configurations and/or settings so that different parts and documentation are needed for the various planes being serviced by the same installation and/or maintenance organizations. Accordingly, it is desirable to provide a cable loss compensation system that avoids the need for different attenuation and compensation devices. In addition, it is desirable that cable loss compensation and/or industry standard loss specifications be achievable with a common system for different aircraft. It is further desirable that the cable loss compensation means and method be capable of automatic operation so that loss compensation is achieved without human intervention. It is additionally desirable that the system be able to compensate in whole or part for changes in cable loss that occur over time due to system aging or other factors. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.